Liquid crystal display device

ABSTRACT

Provided is a liquid crystal display device having a structure in which one surface of the alignment film contacts a liquid crystal layer, the other surface of the alignment film contacts an underlying layer, the refractive index of the alignment film monotonically increases in a film thickness direction of the alignment film from a boundary surface between the liquid crystal layer and the alignment film to a boundary surface between the underlying layer and the alignment film, and the minimum refractive index n LC  of the liquid crystal layer, the refractive index n F2  of the alignment film at the boundary surface between the liquid crystal layer and the alignment film, the refractive index n F1  of the alignment film at the boundary surface between the alignment film and the underlying layer, and the refractive index n S2  of the underlying layer have a relationship of an equation (I) in which n LC ≦n F2 &lt;n F1 ≦n S2 .

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese patent applicationJP2010-058509 filed on Mar. 15, 2010, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device thathas an alignment film with improved transparency and improved highcontrast performance.

2. Description of the Related Art

A liquid crystal display device has been used in various fields due toits features such as high display quality, a thin thickness, a lightweight, and low power consumption. For example, the liquid crystaldisplay device has been used as a monitor for a portable device such asa cellular phone and a digital still camera, a monitor for a desktop PC,a monitor for printing or designing, a monitor for a medical device, anLCD television, and the like.

As the liquid crystal display device has been used in various fields,high image quality and high quality thereof have been demanded.Particularly, high luminance and low power consumption thereof with hightransmissivity have been strongly demanded. Further, there is a strongdemand in which the liquid crystal display device needs to be suppliedto the market at a low price.

Generally, in the liquid crystal display device, an alignment directionof a liquid crystal molecule changes when applying an electric field tothe liquid crystal molecule of a liquid crystal layer interposed betweena pair of substrates, whereby optical characteristics of the liquidcrystal layer change, so that an image is displayed on the liquidcrystal display device. When there is no application of an electricfield, the alignment direction of the liquid crystal molecule isdetermined by an alignment film obtained by performing a rubbingtreatment on a surface of a polyimide thin film.

In the active driving type liquid crystal display device having aswitching element such as a thin film transistor (TFT) for each pixel,an electrode is provided in each of a pair of substrates sandwiching aliquid crystal layer, a so-called vertical electric field is set so thatthe direction of the electric field applied to the liquid crystal layeris substantially perpendicular to a surface of the substrate, and animage is displayed on the liquid crystal display device by using anoptical rotary power of a liquid crystal molecule forming the liquidcrystal layer.

As the representative the vertical electric field system type liquidcrystal display device, a twisted nematic (TN) type is known. In the TNtype liquid crystal display device, a narrow viewing angle is one of anumber of problems. Therefore, an IPS (In-Plane Switching) type or anFFS (Fringe-Field Switching) type has been introduced into the market asa display type of realizing a wide viewing angle.

Each of the IPS type and the FFS type is a so-called horizontal electricfield type in which a pectinate electrode is formed at one of a pair ofsubstrates, and a generated electric field is substantially parallel tothe surface of the substrate. Here, a liquid crystal molecule formingthe liquid crystal layer is rotated within a plane substantiallyparallel to the substrate, and an image is displayed by usingbirefringence of the liquid crystal layer. This type has benefits that aviewing angle is wider than that of the TN type due to the in-planeswitching of the liquid crystal molecule and load capacity is lower thanthat of the TN type. Due to such benefits, the horizontal electric fieldtype has been expected to be a new liquid crystal display device whichmay be used instead of the TN type, and has been rapidly developed inrecent years.

The liquid crystal display element controls the alignment state of theliquid crystal molecule inside the liquid crystal layer by the presenceof the electric field. That is, upper and lower polarizers providedoutside the liquid crystal layer are disposed to be completelyperpendicular to each other, and a phase difference is generated by thealignment state of the liquid crystal molecule therebetween, therebyforming light and dark state.

The transmissivity of the liquid crystal display element is largelydependent on not only light absorbing or scattering of various opticalthin films such as a substrate, a transparent electrode, a liquidcrystal layer, and a polarizer, but also light reflecting at a boundarysurface originating from a difference in refractive index betweenoptical thin films. The maximum refractive index may be obtained insidethe optical thin film of the liquid crystal display element by indiumtin oxide (ITO) having a refractive index set to 2.1 and used in atransparent electrode or silicon nitride (SiNx) having a refractiveindex set from 1.8 to 1.9 used in an interlayer isolation film thatelectrically isolates a pixel electrode and a common electrode from eachother. Examples of the other members include an organic optical thinfilm such as a liquid crystal, an alignment film, a polarizer, or aretardation film, or a glass substrate, and the refractive index thereofis from about 1.4 to about 1.6.

The reflection loss between media having different refractive indexesmay be effectively reduced by inserting various reflections preventinglayers therebetween. As a representative reflection preventing layer, amulti-layer film such as a high refractive index layer and a lowrefractive index layer, a micro-lens array, or the like is used.However, since at least an optical structure having a wavelength orderneeds to be provided, it is difficult to provide the reflectionpreventing layer at a gap of about 100 nm at most between thetransparent electrode and the liquid crystal layer of the liquid crystaldisplay device. In order to make the thin film thinner and to equallyprevent the reflection throughout the visible range, a grated refractiveindex (Grated Refractive Index, GRIN) thin film is used between theoptical thin films having different refractive indexes to smoothlychange the refractive index.

For example, the GRIN thin film is used to prevent connection loss at anoptical communication fiber connection portion formed of inorganic glass(refer to Technical document 1: “Opt. Commun. 2002 28th Euro. Conf. Opt.Commun. (ECOC2002) 3 (2002) 1-2”), and is used in an external lightprevention film formed of SiO₂ glass in which the concentration of holesis controlled (refer to Technical document 2: “Appl. Opt. 42 (2003)4573-4579”).

Further, JP 2007-248607 A suggests a structure in which a GRIN thin filmformed of a SiOx thin film having a controlled oxidation degree is usedin a liquid crystal display element.

SUMMARY OF THE INVENTION

However, since only the external light reflection prevention film formedof SiO₂ glass or only the GRIN thin film formed of a SiOx thin film doesnot exhibit any in-plane anisotropy, it is very difficult to obtainliquid crystal alignment capability. Further, in the structure disclosedin JP 2007-248607 A, there is an attempt to make slight liquid crystalalignment capability by performing oblique vapor deposition using a SiOxthin film alone. However, it is generally known that the oblique vapordeposition film formed of an inorganic material has insufficient liquidcrystal alignment capability.

Nowadays, in order to align the liquid crystal molecule, an alignmentfilm formed of an organic material capable of aligning the liquidcrystal molecule by interaction at the molecular level is generallyused. If the existing liquid crystal alignment film is coated onto theGRIN thin film formed of a SiOx thin film shown in JP 2007-248607 A, adifference in refractive index may be alleviated. However, for example,since a distance from the transparent electrode to the liquid crystallayer becomes longer, the liquid crystal display device is not drivenwhen a voltage necessary for aligning the liquid crystal molecule is notincreased, and power consumption increases markedly.

An object of the present invention is to provide a liquid crystaldisplay device having high transmissivity without degrading a low powerconsumption property or an alignment property of a liquid crystal.Further, the above-mentioned object and other objects, and novelfeatures of the present invention are clarified by the descriptions andthe attached drawings of the specification.

According to an aspect of the present invention, there is provided aliquid crystal display device including: first and second substrates, atleast one of which is transparent; a liquid crystal layer which isdisposed between the first and second substrates; an electrode groupwhich is formed on at least one of the first and second substrates andapplies an electric field to the liquid crystal layer; a plurality ofactive elements which is connected to the electrode group; an alignmentfilm which is disposed on at least one of the first and secondsubstrates and has one surface contacting the liquid crystal layer; andan underlying layer which is disposed on at least one of the first andsecond substrates and contacts the other surface of the alignment film,wherein the alignment film includes an organic compound, wherein therefractive index of the alignment film monotonically increases from aboundary surface between the alignment film and the liquid crystal layerto a boundary surface between the alignment film and the underlyinglayer, and wherein the minimum refractive index n_(LC) of the liquidcrystal layer, the refractive index n_(F2) of the alignment film at theboundary surface between the liquid crystal layer and the alignmentfilm, the refractive index n_(F1) of the alignment film at the boundarysurface between the alignment film and the underlying layer, and therefractive index n_(S2) of the underlying layer have a relationship ofan equation (I) in which n_(LC)≦n_(F2)<n_(F1)≦n_(S2).

Further, the underlying layer may be a transparent layer through whichvisible light is transmitted. The electrode group may include a commonelectrode and a pixel electrode. A surface opposite to a surfacecontacting the alignment film in the underlying layer may contact anyone of the common electrode and the pixel electrode.

The refractive index n_(S2) of the underlying layer and the refractiveindex n_(PE) of the one electrode at the boundary surface between theunderlying layer and the one electrode may have a relationship of anequation (II) in which |n_(S2)−n_(PE)|≦0.1.

A part of the surface opposite to the surface contacting the alignmentfilm in the underlying layer may contact the one electrode and the otherpart thereof contacts an interlayer isolation film. The refractive indexn_(S2) of the underlying layer and the refractive index n_(LI) of theinterlayer isolation film may have a relationship of an equation (III)in which |n_(S2)−n_(LI)|≦0.1.

The electrode group may include a common electrode and a pixelelectrode. The underlying layer may include an interlayer isolation filmcontacting a surface opposite to a surface contacting the liquid crystallayer in the alignment film and any one of the common electrode and thepixel electrode provided in a part of a surface on the side of thealignment film in the interlayer isolation film and protruding towardthe alignment film. The alignment film may be formed on the underlyinglayer. The film thickness of the alignment film may be larger than thatof the one electrode.

The refractive index of the surface of the one electrode may be lowerthan the internal refractive index. The alignment film may have anin-plane optical anisotropy. The direction of the in-plane opticalanisotropic axis of the alignment film may be equal to the alignmentregulation direction of the liquid crystal layer.

The alignment film may include a plurality of types of the organiccompounds. The alignment film includes a plurality of types of theorganic compounds having different polarities. The alignment filmincludes a plurality of types of the organic compounds having differentrefractive indexes. In the plurality of types of organic compoundsforming the alignment film, the refractive index in the visible range ofthe organic compound having the highest refractive index may be 1.7 ormore.

The alignment film may be formed of polyimide using polyamic acid esteras a precursor. The alignment film may obtain liquid crystal alignmentcapability by photo-alignment treatment. The alignment film may obtainliquid crystal alignment capability by a rubbing treatment.

Further, an area obtaining liquid crystal alignment capability of thealignment film may be within a range of 20 nm from a surface of thealignment film. Compounds forming the alignment film may be cross-linkedto each other after the alignment film obtains liquid crystal alignmentcapability. The coating ratio with respect to a display area of thealignment film may be 50% or more.

According to the present invention, the liquid crystal display devicemay be provided which has high transmissivity without degrading a lowpower consumption property or an alignment property of a liquid crystal.The other effects of the present invention are clarified from the entiredescription of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic block diagram illustrating an example of aschematic configuration of a liquid crystal display device according toan embodiment of the present invention.

FIG. 1B is a schematic circuit diagram illustrating an example of acircuit configuration of one pixel of a liquid crystal display panel inthe liquid crystal display device according to the embodiment of thepresent invention.

FIG. 1C is a schematic plan view illustrating an example of a schematicconfiguration of a liquid crystal display panel in the liquid crystaldisplay device according to the embodiment of the present invention.

FIG. 1D is a schematic cross-sectional view illustrating an example of across-sectional configuration taken along the line 1D-1D of FIG. 1C.

FIG. 2A is a schematic cross-sectional view illustrating an example of aschematic configuration of an IPS type liquid crystal display panel inthe liquid crystal display device according to the embodiment of thepresent invention.

FIG. 2B is a schematic cross-sectional view illustrating an example of aschematic configuration of the IPS type liquid crystal display panel inanother example of the liquid crystal display device according to theembodiment of the present invention.

FIG. 3 is a schematic cross-sectional view illustrating an example of aschematic configuration of an FFS type liquid crystal display panel inthe liquid crystal display device according to the embodiment of thepresent invention.

FIG. 4 is a schematic cross-sectional view illustrating an example of aschematic configuration of a VA type liquid crystal display panel in theliquid crystal display device according to the embodiment of the presentinvention.

FIG. 5A is a schematic view illustrating an example of a structure inthe vicinity of an alignment film in the liquid crystal display deviceaccording to the embodiment of the present invention.

FIG. 5B is an explanatory diagram illustrating an example of aconcentration distribution of a chemical structure D included in thealignment film provided in the liquid crystal display device accordingto the embodiment of the present invention.

FIG. 5C is an explanatory diagram illustrating an example of arefractive index distribution in the vicinity of the alignment filmprovided in the liquid crystal display device according to theembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described in detail byreferring to the accompanying drawings. Further, in all drawings used toillustrate the embodiment, the same reference numerals will be given tothe components having the same function, and the description thereofwill not be repeated.

First, an example of a schematic configuration of a liquid crystaldisplay device according to the present invention will be described byreferring to FIGS. 1A to 1D.

FIG. 1A is a schematic block diagram illustrating an example of aschematic configuration of a liquid crystal display device according toan embodiment of the present invention. FIG. 1B is a schematic circuitdiagram illustrating an example of a circuit configuration of one pixelof a liquid crystal display panel in the liquid crystal display deviceaccording to the embodiment of the present invention. FIG. 1C is aschematic plan view illustrating an example of a schematic configurationof a liquid crystal display panel in the liquid crystal display deviceaccording to the embodiment of the present invention. FIG. 1D is aschematic cross-sectional view illustrating an example of across-sectional configuration taken along the line 1D-1D of FIG. 1C.

The present invention is applied to, for example, an active matrixliquid crystal display device. The active matrix liquid crystal displaydevice is used in, for example, a display (monitor) for a portableelectronic apparatus, a display for a personal computer, a display forprinting or designing, a display for a medical device, a liquid crystaltelevision, and the like.

For example, as shown in FIG. 1A, the active matrix liquid crystaldisplay device includes a liquid crystal display panel 1, a firstdriving circuit 2, a second driving circuit 3, a control circuit 4, abacklight 5, and the like.

The liquid crystal display panel 1 includes plural scanning signal linesGL (gate lines) and plural video signal lines DL (drain lines), wherethe video signal lines DL are connected to the first driving circuit 2,and the scanning signal lines GL are connected to the second drivingcircuit 3. Further, FIG. 1A shows a part of the plural scanning signallines GL, but in the actual liquid crystal display panel 1, variousscanning signal lines GL are further densely disposed. In the samemanner, FIG. 1A shows a part of the plural video signal lines DL, and inthe actual liquid crystal display panel 1, various video signal lines DLare further densely disposed.

A display area DA of the liquid crystal display panel 1 includes pluralpixels. An area of one pixel in the display area DA corresponds to, forexample, an area surrounded by two adjacent scanning signal lines GL andtwo adjacent video signal lines DL. In this case, each pixel has, forexample, a circuit configuration shown in FIG. 1B, and includes a TFTelement Tr serving as an active element, a pixel electrode PX, a commonelectrode CT (which may be called a counter electrode), and a liquidcrystal layer LC. Here, for example, a common line CL used to share thecommon electrode CT of the plural pixels is provided in the liquidcrystal display panel 1.

For example, as shown in FIGS. 1C and 1D, the liquid crystal displaypanel 1 has a structure in which alignment films 606 and 705 arerespectively formed on surfaces of an active matrix substrate 6 and acounter substrate 7, and liquid crystal layer 11 a (liquid crystalmaterial) is disposed between the alignment films. Further, although itis not particularly shown herein, an intermediate layer (for example, anoptical intermediate layer such as a light diffusion layer, a colorconversion layer, and a retardation film) may be appropriately providedbetween the alignment film 606 and the active matrix substrate 6 orbetween the alignment film 705 and the counter substrate 7. Here, theactive matrix substrate 6 and the counter substrate 7 are bonded to eachother by an annular sealing agent 8 provided outside the display areaDA, and the liquid crystal layer 11 a is sealed in a space surrounded bythe alignment film 606 on the side of the active matrix substrate 6, thealignment film 705 on the side of the counter substrate 7, and thesealing agent 8. In this case, the liquid crystal display panel 1 of theliquid crystal display device including the backlight 5 includes a pairof polarizers 9 a and 9 b that are disposed to face each other with theactive matrix substrate 6, the liquid crystal layer 11 a, and thecounter substrate 7 interposed therebetween.

Incidentally, the active matrix substrate 6 is a substrate in which thescanning signal lines GL, the video signal lines DL, the active elements(TFT elements Tr), the pixel electrodes PX, and the like are disposed onan isolation substrate such as a glass substrate. When the liquidcrystal display panel 1 is of a horizontal electric field driving typesuch as an IPS type, the common electrode CT and the common line CL aredisposed on the active matrix substrate 6.

Further, when the liquid crystal display panel 1 is a vertical electricfield driving type such as a TN type or a VA (Vertically Alignment)type, the common electrode CT is disposed on the counter substrate 7. Inthe case of the vertical electric field driving type liquid crystaldisplay panel 1, the common electrode CT is generally formed as oneplane-shaped electrode that has a large area and is shared by allpixels, and the common line CL is not provided thereon.

Further, in the liquid crystal display device according to the presentinvention, for example, plural columnar spacers 10 are provided in thespace where the liquid crystal layer 11 a is sealed in order to equalizethe thickness (which may be called a cell gap) of the liquid crystallayer 11 a for each pixel. The plural columnar spacers 10 are providedon, for example, the counter substrate 7.

The first driving circuit 2 is a driving circuit that generates a videosignal (which may be called a grayscale voltage) applied to the pixelelectrode PX of each pixel via the video signal lines DL, and also adriving circuit generally called a source driver, a data driver, and thelike. Further, the second driving circuit 3 is a driving circuit thatgenerates a scanning signal applied to the scanning signal lines GL, andalso a driving circuit generally called a gate driver, a scanningdriver, and the like. The control circuit 4 is a circuit that controlsthe operation of the first driving circuit 2, the operation of thesecond driving circuit 3, and the luminance of the backlight 5, and alsoa control circuit generally called a TFT controller, a timingcontroller, and the like.

Further, the backlight 5 is, for example, alight source such as a lightemitting diode (LED) or a fluorescent light such as a cold cathodefluorescent light, and light emitted from the backlight 5 is convertedinto planar light by a reflection plate, a light guiding plate, a lightdiffusion plate, a prism sheet, and the like which are not shown in thedrawings to be emitted to the liquid crystal display panel 1. When lightis emitted from the backlight 5, the light passes through various layersprovided in the liquid crystal display panel 1.

The liquid crystal display device according to the embodiment of thepresent invention is a liquid crystal display device that includes: theactive matrix substrate (the first substrate) 6 and the countersubstrate (the second substrate) 7, anyone of the first and secondsubstrates formed to be transparent; the liquid crystal layer 11 adisposed between the active matrix substrate 6 and the counter substrate7; the electrode groups CT and PX formed on at least one of the activematrix substrate 6 and the counter substrate 7 to apply an electricfield 12 to the liquid crystal layer 11 a; the plural active elements Trconnected to the electrode groups CT and PX; the alignment films 606 and705 disposed on at least one of the active matrix substrate 6 and thecounter substrate 7 and contacting the liquid crystal layer 11 a at onesurface thereof; and an underlying layer contacting the other surfacesof the alignment films 606 and 705 in at least one of the active matrixsubstrate 6 and the counter substrate 7.

Reflection loss occurs at a boundary surface when the light emitted fromthe backlight 5 is transmitted through the boundary surface of twolayers having different refractive indexes. The reflection loss becomesmore prominent as a difference in refractive index becomes larger.Particularly, the reflection loss becomes prominent when the reflectionloss is caused by the reflection from the boundary surface of theinterlayer isolation film or the electrode groups CT and PX having highrefractive index.

For example, when the liquid crystal display device according to theembodiment of the present invention is a so-called horizontal electricfield driving type liquid crystal display device such as an IPS, theelectrode groups CT and PX are formed only on one substrate (the activematrix substrate 6). In this case, the reflection loss in the activematrix substrate 6 may be effectively suppressed when the alignment filmaccording to the present invention is applied to the alignment film 606provided in the active matrix substrate 6.

Further, in the case of a vertical electric field driving type liquidcrystal display device, one electrode group (the pixel electrodes PX)and the other electrode group (the common electrodes CT) arerespectively formed on the active matrix substrate 6 and the countersubstrate 7. In this case, the alignment film according to the presentinvention may be effectively applied to the alignment films 606 and 705respectively provided in the active matrix substrate 6 and the countersubstrate 7.

In the following description, when the liquid crystal display deviceaccording to the embodiment of the present invention is the horizontalelectric field driving type liquid crystal display device, the alignmentfilm 606 provided in the active matrix substrate 6 will be described indetail. Incidentally, even in the horizontal electric field driving typeliquid crystal display device, the alignment according to the presentinvention may be applied to the alignment film 705 provided in thecounter substrate 7.

Further, when the liquid crystal display device according to theembodiment of the present invention is the vertical electric fielddriving type liquid crystal display device, the alignment film accordingto the present invention may be effectively applied to the alignmentfilms 606 and 705 respectively provided on the active matrix substrate 6and the counter substrate 7. However, in the following description, thedetailed description of the configuration of the counter substrate 7will be omitted.

FIG. 2A is a schematic cross-sectional view illustrating an example of aschematic configuration of an IPS type liquid crystal display panel inthe liquid crystal display device according to the embodiment of thepresent invention.

The liquid crystal display device according to the example of FIG. 2A isa liquid crystal display device that includes: the active matrixsubstrate (the first substrate) 6 and the counter substrate (the secondsubstrate) 7, any one of the first and second substrates formed to betransparent; the liquid crystal layer 11 a disposed between the activematrix substrate 6 and the counter substrate 7; the pixel electrode PXand the common electrode CT formed on one (the active matrix substrate6) of the active matrix substrate 6 and the counter substrate 7 to applythe electric field 12 to the liquid crystal layer 11 a; the activeelement Tr connected to the pixel electrode PX; and the alignment films606 and 705 respectively disposed on the active matrix substrate 6 andthe counter substrate 7.

In this example, the alignment film 606 provided in the active matrixsubstrate 6 includes an organic compound.

The refractive index of the alignment film 606 monotonically increasesfrom the boundary surface between the alignment film 606 and the liquidcrystal layer 11 a to the boundary surface between the alignment film606 and the underlying layer. The minimum refractive index n_(LC) of theliquid crystal layer 11 a, the refractive index n_(F2) of the alignmentfilm 606 at the boundary surface between the liquid crystal layer 11 aand the alignment film 606, the refractive index n_(F1) of the alignmentfilm 606 at the boundary surface between the alignment film 606 and theunderlying layer, and the refractive index n_(S2) of the underlyinglayer have a relationship of a following equation (I).

n_(LC)≦n_(F2)<n_(F1)≦n_(s2)  (I)

Here, the underlying layer is a layer that is formed to contact theopposite side of the liquid crystal layer 11 a in the alignment film606. That is, one surface of the alignment film 606 contacts the liquidcrystal layer 11 a, and the other surface thereof contacts theunderlying layer. The underlying layer of the liquid crystal displaydevice shown in FIG. 2A is an interlayer isolation film 605 and thecommon electrode CT. For example, the underlying layer may be formed tospread on one surface of the liquid crystal panel like the interlayerisolation film 605, or may be formed in a specific pattern like thecommon electrode CT.

Further, even when the underlying layer includes at least two types oflayers (for example, the interlayer isolation film 605 and the commonelectrode CT), the refractive index n_(S2) of each of two or more typesof layers forming the underlying layer at the boundary surfacecontacting the alignment film 606 satisfies the relationship of theequation (I).

Further, the alignment films 606 and 705 directly contacting the liquidcrystal layer 11 a are formed of an organic compound having thermalresistance and mainly including polyimide. The liquid crystal materialforming the liquid crystal layer 11 a is formed by using a nematicliquid crystal mixture. When the liquid crystal material is a positiveliquid crystal, the refractive index no in the normal optical directionis about 1.7, and the refractive index ne in the abnormal opticaldirection is about 1.5. On the contrary, when the liquid crystalmaterial is a negative liquid crystal, the no is about 1.5, and the neis about 1.7. The minimum refractive index n_(LC) of the liquid crystallayer shown in the above-described equation (I) indicates the smallervalue among the refractive index no in the normal optical direction ofliquid crystal molecules forming the liquid crystal layer and therefractive index ne in the abnormal optical direction. For example, inthe minimum refractive index n_(LC) of the liquid crystal layer formedof a positive liquid crystal, the refractive index ne in the abnormaloptical direction is 1.5.

The active matrix substrate 6 has a structure in which the scanningsignal lines GL, the common line CL, and a first isolation film 602covering the lines are formed on a surface of an isolation substratesuch as a glass substrate 601. The semiconductor layer 603 of the TFTelement Tr, the video signal lines DL, the pixel electrodes PX, and asecond isolation film 604 covering the layers, the lines, and theelectrodes are formed on the first isolation film 602. The semiconductorlayer 603 is disposed on the scanning signal lines GL, and a portionlocated below the semiconductor layer 603 in the scanning signal linesGL serves as a gate electrode of the TFT element Tr.

Further, the semiconductor layer 603 has, for example, a structure inwhich a source diffusion layer and a drain diffusion layer formed ofsecond amorphous silicon having different types of impurities ordifferent concentrations of impurities from a first amorphous siliconare laminated on an active layer (channel formation layer) formed of thefirst amorphous silicon. In this case, a part of the video signal linesDL and a part of the pixel electrodes PX are disposed on thesemiconductor layer 603, and the portions disposed on the semiconductorlayer 603 respectively serve as the drain electrode and the sourceelectrode of the TFT element Tr.

Here, the source and the drain of the TFT element Tr are switched inaccordance with a relationship of bias, that is, a relationship ofdifference between the potential of the pixel electrode PX and thepotential of the video signal line DL when turning on the TFT elementTr. However, in the following description of the specification, anelectrode connected to the video signal line DL is referred to as adrain electrode, and an electrode connected to the pixel electrode PX isreferred to as a source electrode.

An interlayer isolation film 605 (overcoat layer) having a planarizedsurface is formed on the second isolation film 604. The common electrodeCT and the alignment film 606 covering the common electrode CT and theinterlayer isolation film 605 are formed on the interlayer isolationfilm 605. The common electrode CT is connected to the common line CL viaa contact hole CH (through hole) penetrating the first isolation film602, the second isolation film 604, and the interlayer isolation film605.

Further, as shown in FIG. 2A, the electrode group included in the activematrix substrate 6 includes the common electrode CT and the pixelelectrode PX. The above-described underlying layer includes theinterlayer isolation film 605 contacting the surface opposite to thesurface contacting the liquid crystal layer 11 a of the alignment film606, and the common electrode CT provided in a part of the surface ofthe interlayer isolation film 605 on the side of the alignment film 606to protrude toward the alignment film 606. The alignment film 606 isformed on the underlying layer (the interlayer isolation film 605 andthe common electrode CT), and the film thickness of the alignment film606 is larger than that of the common electrode CT.

When the film thickness of the alignment film 606 is set to be largerthan that of the common electrode CT, unevenness of the common electrodeCT formed on the upper surface of the interlayer isolation film 605 maybe reduced, and hence the reflection loss originated from the unevennessof the common electrode CT may be suppressed, which is desirable in theimprovement in transmissivity.

Further, the common electrode CT is set so that a gap Pg with respect tothe pixel electrode PX in the plan view is set to, for example, about 7μm. A polymer material to be described in the examples below is coatedonto the alignment film 606, and a surface treatment (a rubbingtreatment or the like) is performed on the surface to obtain a liquidcrystal alignment property.

On the other hand, the counter substrate 7 has a structure in which ablack matrix 702, color filters 703R, 703G, 703B, and an overcoat layer704 covering the black matrix and the color filters are formed on asurface of an isolation substrate such as a glass substrate 701. Theblack matrix 702 is, for example, a light shielding film that is formedin a lattice shape and is used to form an opening area by the unit ofpixel in the display area DA. Further, each of the color filters 703R,703G, 703B is a film that allows only the transmission of light having aspecific wavelength area (color) among white light emitted from thebacklight 5. When the liquid crystal display device corresponds to anRGB type color display, the color filter 703R allowing the transmissionof red light, the color filter 703G allowing the transmission of greenlight, and the color filter 703B allowing the transmission of blue lightare disposed (here, the pixel of one color is representatively shown).

Further, the overcoat layer 704 has a planarized surface. The pluralcolumnar spacers 10 and the alignment film 705 are formed on theovercoat layer 704. The columnar spacer 10 has, for example, a topportion that is formed in a flat conical trapezoid shape (which may becalled a trapezoid rotation body), and is formed at a positionoverlapping with a portion except for a portion intersecting the videosignal line DL and the portion having the TFT element Tr among thescanning signal lines GL of the active matrix substrate 6.

Further, the alignment film 705 is formed of, for example, a polyimideresin, and a surface treatment (a rubbing treatment or the like) isperformed on the surface to obtain a liquid crystal alignment property.Further, when there is no application of an electric field in which thepotentials of the pixel electrode PX and the common electrode CT areequal to each other, the liquid crystal molecule 11 b of the liquidcrystal layer 11 a in the liquid crystal display panel 1 of FIG. 2A isaligned to be substantially parallel to the surfaces of the glasssubstrates 601 and 701, and is homogenously aligned in the directionfacing the initial alignment direction defined by the rubbing treatmentperformed on the alignment films 606 and 705.

Then, when the TFT element Tr is turned on to write the grayscalevoltage applied to the video signal line DL into the pixel electrode PX,so that a difference in potential occurs between the pixel electrode PXand the common electrode CT, the electric field 12 (line of electricalforce) is generated as shown in the drawing, and the electric field 12having a strength in accordance with a difference in potential betweenthe pixel electrode PX and the common electrode CT is applied to theliquid crystal molecule 11 b. Here, due to the interaction of theelectric field 12 and dielectric anisotropy of the liquid crystal layer11 a, the direction of the liquid crystal molecule 11 b forming theliquid crystal layer 11 a changes to the direction of the electric field12, so that refraction anisotropy of the liquid crystal layer 11 achanges. In this case, the direction of the liquid crystal molecule 11 bis determined by the strength (the largeness of a difference inpotential between the pixel electrode PX and the common electrode CT) ofthe applied electric field 12. Accordingly, in the liquid crystaldisplay device, for example, the potential of the common electrode CT isfixed, and the grayscale voltage applied to the pixel electrode PX iscontrolled for every pixel to change the light transmissivity of eachpixel, thereby displaying a video or an image.

FIG. 2B is a schematic cross-sectional view illustrating an example of aschematic configuration of the IPS type liquid crystal display panel inanother example of the liquid crystal display device according to theembodiment of the present invention. A transparent layer 610 is formedon the common electrode CT in the liquid crystal display device shown inFIG. 2A, and the alignment film 606 is formed on the transparent layer610. Further, the transparent layer 610 is used to planarize theunevenness caused by the step of the common electrode CT in the displayarea, and suppresses the reflection loss originated from the unevennessof the common electrode CT.

The underlying layer of the liquid crystal display device shown in FIG.2B is the transparent layer 610 through which visible light (anelectromagnetic wave having a wavelength range from 380 nm to 750 nm) istransmitted. The transparent layer 610 contacts the surface opposite tothe surface contacting the liquid crystal layer 11 a of the alignmentfilm 606 in the active matrix substrate 6. A part of the surfaceopposite to the surface contacting the alignment film 606 of theunderlying layer contacts the common electrode CT. Further, therefractive index n_(S2) of the transparent layer 610 as the underlyinglayer and the refractive index n_(PE) of the common electrode CT at theboundary surface between the transparent layer 610 and the commonelectrode CT may have a relationship of a following equation (II).

|n _(s2) −n _(PE)|≦0.1  (II)

When a difference between the refractive index n_(S2) of the transparentlayer 610 as the underlying layer and the refractive index n_(PE) of thecommon electrode CT is 0.1 or less, the reflection loss originated fromthe interlayer reflection between the transparent layer 610 and thecommon electrode CT may be reduced, so that the transmissivity may bedesirably improved. Further, when the refractive index n_(S2) of thetransparent layer 610 is equal to the refractive index n_(PE) of thecommon electrode CT, the reflection loss originated from the interlayerreflection between the transparent layer 610 and the common electrode CTmay be particularly reduced, so that the transmissivity may beparticularly desirably improved.

Incidentally, the refractive index n_(PE) of the common electrode CT atthe boundary surface between the transparent layer 610 and the commonelectrode CT may be set to be lower than the internal refractive index.Further, when the common electrode CT is uniform, the refractive indexof the surface of the common electrode CT may be set to be equal to theinternal refractive index.

In the liquid crystal display device shown in FIG. 2B, a part of thesurface opposite to the surface contacting the alignment film 606 of thetransparent layer 610 as the underlying layer contacts the commonelectrode CT, and the other part contacts the interlayer isolation film605. The refractive index n_(S2) of the transparent layer 610 and therefractive index n_(LI) of the interlayer isolation film 605 may have arelationship of a following equation (III).

|n _(s2) −n _(LI)|≦0.1  (III)

When a difference between the refractive index n_(S2) of the transparentlayer as the underlying layer and the refractive index n_(LI) of theinterlayer isolation film 605 is 0.1 or less, the reflection lossoriginated from the interlayer reflection between the transparent layer610 and the interlayer isolation film 605 may be reduced, so that thetransmissivity may be desirably improved. Further, when the refractiveindex n_(S2) of the transparent layer 610 is equal to the refractiveindex n_(LI) of the interlayer isolation film 605, the reflection lossoriginated from the interlayer reflection between the transparent layer610 and the interlayer isolation film 605 may be particularly reduced,so that the transmissivity may be particularly desirably improved.

FIG. 3 is a schematic cross-sectional view illustrating an example of aschematic configuration of an FFS type liquid crystal display panel inthe liquid crystal display device according to the embodiment of thepresent invention. The liquid crystal display device according to theexample of FIG. 3 is a liquid crystal display device that includes: theactive matrix substrate (the first substrate) 6 and the countersubstrate (the second substrate) 7, any one of the first and secondsubstrates formed to be transparent; the liquid crystal layer 11 adisposed between the active matrix substrate 6 and the counter substrate7; the pixel electrode PX and the common electrode CT formed on one (theactive matrix substrate 6) of the active matrix substrate 6 and thecounter substrate 7 to apply the electric field 12 to the liquid crystallayer 11 a; the active element connected to the pixel electrode; and thealignment films 606 and 705 respectively disposed on the active matrixsubstrate 6 and the counter substrate 7.

The underlying layer of the liquid crystal display device shown in FIG.3 is the second isolation film 604 and the pixel electrode PX. Further,the refractive index n_(S2) of each of the second isolation film 604 andthe pixel electrode PX forming the underlying layer at the boundarysurface contacting the alignment film 606 satisfies the relationship ofthe equation (I).

The active matrix substrate 6 has a structure in which the commonelectrode CT, the scanning signal lines GL, the common line CL, and thefirst isolation film 602 covering the electrodes and the lines areformed on a surface of an isolation substrate such as the glasssubstrate 601. The semiconductor layer 603 of the TFT element, the videosignal line DL, the source electrode, and the second isolation film 604covering the layers, the lines, and the electrodes are formed on thefirst isolation film 602. In this case, a part of the video signal linesDL and a part of the source electrodes are disposed on the semiconductorlayer 603, and the portions disposed on the semiconductor layer 603respectively serve as the drain electrode and the source electrode ofthe TFT element.

Further, in the liquid crystal display panel 1 of FIG. 3, the interlayerisolation film 605 is not provided, and the pixel electrode PX and thealignment film 606 covering the pixel electrode PX are provided on thesecond isolation film 604. The pixel electrode PX is connected to thesource electrode via the contact hole CH (through hole) penetrating thesecond isolation film 604. Here, the common electrode CT formed on thesurface of the glass substrate 601 is formed in a plane shape at an area(opening area) surrounded by two adjacent scanning signal lines GL andtwo adjacent video signal lines DL, and the pixel electrode PX havingplural slits is laminated on the plane-shaped common electrode CT. Inthis case, the common electrode CT of the pixels arranged in theextension direction of the scanning signal lines GL is shared by thecommon line CL.

On the other hand, the counter substrate 7 of the liquid crystal displaypanel 1 of FIG. 3 has the same configuration as that of the countersubstrate 7 of the liquid crystal display panel of FIG. 2. Accordingly,the detailed description of the configuration of the counter substrate 7will not be repeated.

FIG. 4 is a schematic cross-sectional view illustrating an example of aschematic configuration of a VA type liquid crystal display panel in theliquid crystal display device according to the embodiment of the presentinvention.

The liquid crystal display device according to the example of FIG. 4 isa liquid crystal display device that includes: the active matrixsubstrate (the first substrate) 6 and the counter substrate (the secondsubstrate) 7, any one of the first and second substrates formed to betransparent; the liquid crystal layer 11 a disposed between the activematrix substrate 6 and the counter substrate 7; the pixel electrode PXand the common electrode CT respectively formed on the active matrixsubstrate 6 and the counter substrate 7 to apply the electric field 12to the liquid crystal layer 11 a; the active element Tr connected to thepixel electrode PX; and the alignment films 606 and 705 respectivelydisposed on the active matrix substrate 6 and the counter substrate 7.

The underlying layer of the liquid crystal display device shown in FIG.4 is the second isolation film 604 and the pixel electrode PX. Further,the refractive index n_(S2) of each of the second isolation film 604 andthe pixel electrode PX forming the underlying layer at the boundarysurface contacting the alignment film 606 satisfies the relationship ofthe equation (I).

For example, as shown in FIG. 4, the vertical electric field drivingtype liquid crystal display panel 1 has a structure in which the pixelelectrode PX is formed on the active matrix substrate 6, and the commonelectrode CT is formed on the counter substrate 7.

In the VA type liquid crystal display panel 1 as one of the verticalelectric field driving types, each of the pixel electrode PX and thecommon electrode CT is formed in a solid shape (a simple plane shape)by, for example, a transparent conductive material such as an ITO. Inthis case, when there is no application of the potential in which thepotentials of the pixel electrode PX and the common electrode CT areequal to each other, the liquid crystal molecules 11 b are arranged inthe direction perpendicular to the surfaces of the glass substrates 601and 701 by the alignment films 606 and 705.

Then, when a difference in potential occurs between the pixel electrodePX and the common electrode CT, the electric field 12 (line ofelectrical force) is generated so as to be substantially perpendicularto the glass substrates 601 and 701, and the liquid crystal molecules 11b fall down to the direction parallel to the glass substrates 601 and701, so that the polarization state of incident light changes. At thistime, the direction of the liquid crystal molecule 11 b is determined inaccordance with the strength of the applied electric field 12.Accordingly, in the liquid crystal display device, for example, thepotential of the common electrode CT is fixed, and the video signal(grayscale voltage) applied to the pixel electrode PX is controlled forevery pixel to change the light transmissivity of each pixel, therebydisplaying a video or an image.

Further, the configuration of the pixel in the VA type liquid crystaldisplay panel 1, for example, the shape of TFT element or the pixelelectrode PX in the plan view may be variously set as widely known, andthe configuration of the pixel in the liquid crystal display panel 1 ofFIG. 4 may be set as any one of them. Here, the detailed description ofthe configuration of the pixel of the liquid crystal display panel 1will be omitted.

The present invention relates to the liquid crystal display panel 1 inthe above-described active matrix liquid crystal display device, andparticularly, to a configuration of a portion contacting the liquidcrystal panel 11 a in the active matrix substrate 6 and the countersubstrate 7, and the periphery thereof. Therefore, the detaileddescription of the configuration of the first driving circuit 2, thesecond driving circuit 3, the control circuit 4, and the backlight 5 notdirectly involved with the present invention will be omitted.

Here, the components included in the liquid crystal display deviceaccording to the embodiment of the present invention will be described.

As described above, the liquid crystal layer 11 a is filled with aliquid crystal material such as a nematic liquid crystal mixture in manycases. However, in the case of the refractive index of the positiveliquid crystal, the refractive index no in the normal optical directionis about 1.7, and the refractive index ne in the abnormal opticaldirection is about 1.5. On the contrary, in the case of the negativeliquid crystal, the no is about 1.5, and the ne is about 1.7.

Further, the refractive index of polyimide used in the alignment film isabout 1.6. On the contrary, regarding a thin film layer located at thefurther lower side of the alignment film, for example, there is thecommon electrode CT or the pixel electrode PX below the alignment film606 below the liquid crystal layer 11 a in the IPS type liquid crystaldisplay element of FIG. 2A, and the electrodes are isolated by theisolation films 604 and 605.

The common electrode CT or the pixel electrode PX is formed of asputtering film of ITO as a transparent electrode in many cases, and therefractive index thereof is about 2.1. Further, as the isolation films604 and 605, a CVD (Chemical Vapor Deposition) film of SiNx is used inmany cases, and the refractive index thereof is from about 1.8 to about1.9. As the glass substrate therebelow, alkali-free glass is used inmany cases, and the refractive index thereof is about 1.5.

That is, the active matrix substrate 6 has a structure in which a thinfilm layer (the common electrode CT, the isolation film 604, or thelike) having a high refractive index such as to be 1.8 or more isinterposed between thin film layers (the liquid crystal layer 11 a, theglass substrate 601, and the like) having a low refractive index such asto be from 1.5 to 1.7. It is important to suppress the reflection lossoriginated from a difference in interlayer refractive index in order toimprove the transmissivity.

On the other hand, the alignment film 705, the overcoat layer 704, andcolor filters 703R, 703G, 703B are present above the liquid crystallayer 11 a, and all of them are formed of an organic compound, where therefractive index thereof is from about 1.5 to about 1.6. Further, aglass substrate 701 thereabove is formed of alkali-free glass in manycases, and the refractive index thereof is about 1.5. That is, thecounter substrate 7 above the liquid crystal layer is formed as a memberhaving a low refractive index such as to be from 1.5 to 1.6, and thereflection loss originated from a difference in refractive index issmall.

The FFS type liquid crystal display element shown in FIG. 3 hassubstantially the same configuration. Further, in the VA type liquidcrystal display element of FIG. 4, the common electrode CT formed as atransparent electrode is present not only above the liquid crystal layer11 a, but also therebelow. Here, a problem may arise due to thereflection loss originated from a difference in refractive index betweenthe common electrode CT and the overcoat layer 704 or the alignment film705.

FIG. 5A is a schematic view illustrating an example of a structure inthe vicinity of an alignment film in the liquid crystal display deviceaccording to the embodiment of the present invention, which is suitableto solve such a problem.

One side of the alignment film 606 (or 705) is provided with the liquidcrystal layer 11 a, and the other side thereof is provided with thecommon electrode CT or the pixel electrode PX. The alignment film 606(or 705) of the liquid crystal may be desirably formed of arepresentative organic compound such as polyimide from the viewpointthat the alignment regulation force is obtained. Generally, therefractive index of the alignment film including the organic compound isabout 1.6. That is, there is only a difference of about 0.1 with respectto the refractive index of the liquid crystal layer 11 a, but there is adifference of about 0.5 with respect to the ITO used in the commonelectrode CT or the pixel electrode PX.

In the liquid crystal display device according to the embodiment of thepresent invention, a chemical structure D is adopted in the alignmentfilm 606 (or 705) including the organic compound in order to improve therefractive index. For example, as shown in FIG. 5D, the concentration ofthe chemical structure D is set to be high (concentration C₀) at theposition z=0 (the boundary surface between the alignment film 606 (or705) and the liquid crystal 11 a) in the film thickness direction, andset to be low (concentration C_(d)) at the position z=d (the boundarysurface between the alignment film 606 (or 705) and the common electrodeCT (or the pixel electrode PX)) in the film thickness direction, therebyforming the alignment film 606 (or 705) having a grated refractive indexdistribution layer without degrading the alignment property of theliquid crystal. Incidentally, the film thickness direction indicates thedirection straight to the surface of the alignment film, and is alignedfrom one alignment film 606 (or 705) to the other alignment film 705 (or606).

In the alignment film having the grated refractive index distributionlayer, for example, as shown in FIG. 5C, the minimum refractive indexn_(LC) of the liquid crystal layer 11 a, the refractive index n_(F2) ofthe alignment film 606 at the boundary surface between the liquidcrystal layer 11 a and the alignment film 606, the refractive indexn_(F1) of the alignment film 606 at the boundary surface between thealignment film 606 and the underlying layer, and the refractive indexn_(S2) of the underlying layer at the boundary surface between thealignment film 606 and the underlying layer have the relationship of theequation (I).

Further, in the alignment film 606, the refractive index monotonicallyincreases in the film thickness direction of the alignment film 606 fromthe boundary surface between the liquid crystal layer 11 a and thealignment film 606 to the boundary surface between the alignment film606 and the underlying layer. Further, in the alignment film 606, it ismore desirable that the refractive index monotonically and continuouslyincreases in the film thickness direction of the alignment film from theboundary surface between the liquid crystal layer 11 a and the alignmentfilm 606 to the boundary surface between the alignment film 606 and theunderlying layer.

In order to realize the grated refractive index distribution, it isdesirable that the refractive index distribution is smoothly modulatedfrom z=0 to z=d, and this is realized by smoothly modulating theconcentration of the chemical structure D.

Hereinafter, an alignment film including polyimide with the chemicalstructure D will be described in detail. The chemical structure D needsto increase the refractive index of the alignment film while maintainingall characteristics of the alignment film of the liquid crystal displayelement, that is, the characteristics of the existing alignment filmsuch as an alignment regulation force of liquid crystal molecules,reliability at the time of driving for a long period of time, andtransparency. That is, in the liquid crystal display device having athin film layer with a high refractive index set to be 1.8 or moreinterposed therein, the alignment film needs to have a portion with ahigh refractive index set to be at least 1.7 or more and a portion witha low refractive index set to be from 1.5 to 1.6. As a means forrealizing polymer having a high refractive index, for example, in thecase of polyimide polymer, the means may be realized by using polyimideincluding plural sulfur atoms S in a molecular frame introduced in, forexample, Technical document 3 below.

Technical document 3: Recent advancement of polyimide 2009, polyimidestudy committee, association for textile industrial technology, 90-92(2008)

Specifically, a chemical structure may be exemplified which includesvarious sulfur atoms S, such as sulfide (—S—), sulfoxide (—SO—), sulfone(—SO₂—), or sulfonic acid (—SO₃H), sulfinic acid (—SO₂H), and estersthereof.

Alternatively, a chemical structure may be exemplified which includesselenium Se as a heavier atom, such as selenide (—Se—), selenoxide(—SeO—), selenon (—SeO₂—), or selenonic acid (—SeO₃H), selenic acid(—SeO₂H), and esters thereof.

Alternatively, a chemical structure may be exemplified which includestellurium Te as a heavier atom, such as telluride (—TeO—), telluroxide(—TeO—), telluron (—TeO₂—), or telluronic acid (—TeO₃H), tellurinic acid(—TeO₂H), and esters thereof.

Further, a polymer having a high refractive index may be formed in sucha manner that a terminally modified group adopts a heavy halogen atom,for example, chlorine-Cl, bromine-Br, iodine-I, and the like. The highrefractive index may be obtained by using the chemical structure Dcapable of contributing to an increase in refractive index as much aspossible. However, it is necessary to determine the optimal structurewhile seeking compatibility of other material properties such as thelight absorbing property of the material or the liquid crystal moleculealignment capability.

In order to realize a polymer having a higher refractive index, apolymer may be used by dispersing or coordinate bonding transparentinorganic particles having a high refractive index, for example,nanoparticles equal to or smaller than the wavelength size of light suchas ITO, zinc oxide ZnO, titanium oxide TiO₂, or zirconium oxide ZrO₂,but the polymer may be used only on the condition that other propertiesof the alignment film of the liquid crystal display element are notdegraded.

On the other hand, in an organic compound having a low refractive index,polyimide used in the existing liquid crystal display element may beused, and various types of polyimides disclosed in, for example,Japanese Patent No. 4052308 may be used.

By mixing the organic component having a high refractive index and theorganic compound having a low refractive index at an appropriate ratio,an organic compound having an intermediate refractive index may berealized. The refractive index of the organic compound of each singlecomponent may be evaluated by a method of forming each single thin filmand evaluating the refractive index of the thin film by an existingmethod, for example, an Abbe refractometer, a prism coupling method, orthe like.

As for the intermediate refractive index, a thin film is formed byuniformly mixing both organic compounds, and the refractive index isdirectly evaluated by the same method as described above. Also, theconcentration of the element included in the original chemical structureD of the organic compound having a high refractive index, for example, Sor the like is carefully observed, a correlation table between theconcentration of the element and the refractive index of the thin filmis created, and then the refractive index having an arbitraryconcentration may be inserted (in many cases, the refractive index ofthe organic compound may be added). Alternatively, the distribution ofthe refractive index may be checked in such a manner that both endportions of the alignment film are cut, light is emitted to one endportion of the alignment film in the direction perpendicular to the filmthickness direction, and the radiation of the light emitted from theother end portion is observed.

Even when the polymers having a high refractive index and a lowrefractive index are formed of organic compounds, the refractive indexdistribution needs to be realized in the film thickness direction of thealignment film having a film thickness of about 100 nm at most. Forexample, plural mixtures of organic compounds having an intermediaterefractive index between the high refractive index and the lowrefractive index of the polymers are prepared, and are sequentiallylaminated (coated) from the side having the high refractive index,whereby a multi-layer having a gradually changing refractive index maybe realized. However, when the total film thickness is 100 nm, the filmthickness for each layer is equal to or less than 10 nm, and the filmmay not be uniformly formed. Further, the manufacturing cost markedlyincreases with plural times of lamination. If it is possible, it isdesirable that the concentration of the chemical structure D naturallychanges in the film thickness direction of the thin film formed bycoating once.

In the case of a mixture including plural organic compounds, a phaseseparation structure may be naturally obtained due to a difference inmolecular weight of each component or a difference in compatibilitythereof, but this may not guarantee that a desired refractive indexdistribution is obtained in the film thickness direction.

Alternatively, a combination of organic compounds may be selected byusing a difference in specific solubility in such a manner that anorganic compound easily precipitated at an early timing is graduallyseparated toward the underlying layer and an organic compound hardlyprecipitated is gradually separated toward the liquid crystal layer.

Alternatively, a structure of naturally separating the components in thefilm thickness direction during the coating and drying process may beformed in such a manner that one organic compound adopts a polar group,for example, a hydroxyl group (—OH), a carboxylic acid group (—COOH),sulfonic acid group (—SO₃H), an amino group (−NH₂), a nitro group(−NO₃), a cyano group (—CN), or the like, the polarity is appropriatelymaintained, and an organic compound having insufficient polarity iscombined. In this case, a large difference in polarity may be generatedto a certain degree, or an optimal difference in polarity may begenerated by, for example, a largeness of a molecular polarization of apolarity group alone or an introduction concentration thereof. Inaddition, a uniformly mixed solution needs to be formed before thecoating, but this may be realized by, for example, solubility of a mixedsolvent obtained by combining N-methylpyrrolidone (NMP) and γ-butyllactone and optimization of the polarity ratio. For example, when acomponent having high polarity needs to be first precipitated toward theunderlying layer, for example, in the case of the underlying layerformed of ITO, the precipitation may be realized by performing a surfacecleaning treatment, a UV/O₃ treatment, a O₂ plasma treatment, or thelike thereon to improve a hydrophilic property. On the contrary, when acomponent having low polarity needs to be first precipitated toward theunderlying layer, the precipitation may be realized by performing asurface treatment, a SF₄ plasma treatment, a silane coupling agenttreatment, or the like thereon to improve a hydrophobic property.

Further, in order to synthesize polyimide, polyamide acid beforeimidization, or polyamic acid ester having such characteristics, thesynthesis may be performed using general aromatic polyimide, and forexample, the synthesis may be performed by reacting pyromelliticdianhydride and p-phenylenediamine in an organic solvent. Among them,when polyamic acid ester is used as a precursor before imidization,there is an advantage that the reverse progress of imidization may besuppressed.

Alternatively, polyimide having a large molecular weight may beeffectively obtained in such a manner that a precursor having asufficiently large molecular weight is formed by synthesizing polyamicacid ester, and imidization is performed after ester dissociationreaction or vice versa. That is, it is appropriate that the alignmentfilm 606 is formed of polyimide using polyamic acid ester as aprecursor.

Further, as a method of coating the alignment films 606 and 705 formedof polyimide of the present invention onto various substrates, a generalpolyimide alignment film forming method may be used. For example, asolution (alignment film varnish) obtained by dissolving polyimideresin, polyamide acid as a precursor thereof, or polyamic acid ester ina predetermined solvent is coated by a spin coating method, theresultant object is heated at a predetermined condition to evaporate thesolvent, and imidization progresses, thereby forming a thin film. Then,various alignment treatments, for example, a rubbing treatment usingphysical friction of smooth cloth, or a so-called photo-alignmenttreatment by emitting UV light to an alignment film material having aphotoreactive group is performed on the surface of the polyimide thinfilm, thereby exhibiting liquid crystal alignment capability as thealignment film in the liquid crystal display element.

Further, the alignment film 606 may have in-plane optical anisotropy. Asa method of obtaining optical anisotropy on the surface of the alignmentfilm (inside the alignment film), the above-described photo-alignmenttreatment may be exemplified. Alternatively, for example, a method maybe exemplified which stretches the alignment film 606 in one directionor applies a strong magnetic field thereto. Since optical anisotropy isobtained on the surface of the alignment film 606, the image quality ofthe liquid crystal display device may be improved. Also, since thedirection of the optical anisotropic axis on the surface of thealignment film 606 is equal to the alignment regulation direction of theliquid crystal layer 11 a, the transmission loss of the light originatedfrom anisotropy may be suppressed, which is desirable in the improvementin transmissivity.

Further, the refractive index of the surface of the common electrode CTand/or the pixel electrode PX may be lower than the internal refractiveindex. For example, the surface of the common electrode CT becomesoxidized by performing an O₂ plasma treatment or the like on thesurface. In this case, the refractive index of the surface of the commonelectrode CT becomes smaller than the internal refractive index thereof.As described above, the common electrode CT or the pixel electrode PX isformed of a sputtering film of ITO as a transparent electrode in manycases, and the refractive index thereof is as high as about 2.1. Therefractive index of a portion contacting the common electrode CT or thepixel electrode PX is lower than that of the common electrode CT or thepixel electrode PX. When a difference in refractive index is large, adifference in refractive index may be decreased by performing theabove-described treatment, which is desirable in the improvement intransmissivity.

Further, in the liquid crystal display device according to theembodiment of the present invention, an area having the liquid crystalalignment capability of the alignment film 606 may be within the rangeof 20 nm from the surface of the alignment film 606. Further, the areahaving the liquid crystal alignment capability of the alignment film 606may be provided in the range of 20 nm from the surface of the alignmentfilm. When a structure is formed in which the liquid crystal alignmentcapability is provided up to the deeper position, a problem may arise inthat the mechanical strength of the entire alignment film is degraded.The degradation of the mechanical strength of the alignment film mayoccur when the liquid crystal display element is driven for a longperiod of time. That is, the initial alignment direction of the surfaceof the alignment film gradually vanishes, and the liquid crystalalignment capability is degraded, thereby degrading displaycharacteristics.

In order to prevent such degradation, a display degradation may beeffectively prevented by increasing a mechanical strength in such amanner that the alignment film obtains the liquid crystal alignmentcapability and chemical cross-linking is performed. That is, in theliquid crystal display device according to the embodiment of the presentinvention, the alignment film 606 may be formed by cross-linkingcompounds forming the alignment film 606 after obtaining the liquidcrystal alignment capability.

Accordingly, the alignment film 606 obtaining the liquid crystalalignment capability has a cross-linking group, and may be appropriatelysubjected to a cross-linking treatment. For example, when the liquidcrystal alignment capability is obtained by emitting the above-describedUV light thereto, if X shown in the chemical formula (1) to show asfollows has a cyclobutane group, the cyclobutane group is cleaved due tothe emission of UV light, so that a maleimide group is formed. Thecompounds forming the alignment film are cross-linked to each other bythe maleimide group. Further, since the compounds shown in the chemicalformula (1) have a thermal reaction group such as an epoxy group, thecompounds forming the alignment film are cross-linked to each other bythe epoxy group.

In the liquid crystal display device according to the embodiment of thepresent invention, the coating ratio with respect to the display area ofthe alignment film 606 may be 50% or more. That is, since the reflectionloss may be effectively suppressed when the coating ratio of thealignment film with respect to the display area of the liquid crystaldisplay device is 50% or more, the transmissivity may be desirablyimproved.

Further, it is desirable that the coating ratio with respect to thedisplay area of the alignment film is 60% or more, and it is moredesirable that the coating ratio with respect to the display area of thealignment film is 75% or more.

Whether the alignment film mainly including an organic compound has adesired distribution structure of the chemical structure D may bechecked by the following method. First, the uniformity of the thin filmmay be checked on the basis of whether the thin film is uniform or adomain structure is formed by observing the inside of the surface of thefilm using a microscope, a SEM (Scanning Electron Microscopy), an AFM(Atomic Force Microscopy), or the like.

Here, in order to see the uniformity of the distribution state of thechemical structure D within the plane, for example, the original sulfuratom S in the chemical structure D is mainly observed, and in-planeelemental mapping is performed by a SEM-EDX (Energy Dispersive X-rayspectrometry) or the like. As for the thin film structure in the filmthickness direction, the wall of the thin film is opened or thecross-sectional surface of the film is exposed by a FIB (Focused IonBeam Etching) or the like, and the presence of the domain structure ischecked by observation using a SEM and a TEM (Transmission ElectronMicroscopy).

Further, as for the concentration distribution in the film thicknessdirection of the chemical structure D, for example, the original sulfuratom S in the chemical structure D is mainly observed, and thedistribution is checked by an auger spectral analysis during an ion-beamsputtering. By the use of various analysis methods, whether the obtainedthin film is uniform without phase dissociation or whether the chemicalstructure D has a desired concentration distribution may be checked.Then, the refractive index distribution in the film thickness directionmay be estimated on the basis of the concentration distribution of thechemical structure D.

Further, the refractive index of each of the lowermost portion and theuppermost portion of the alignment film formed of the polymer mixture tohave the concentration distribution of the chemical structure D in thefilm thickness direction may be checked by the following method. Forexample, when the underlying layer of the alignment film is atransparent electrode ITO, a solid film of ITO is formed on a glasssubstrate having the same refractive index as that of ITO by the sameprocess, and the surface treatment as that of the actual liquid crystaldisplay device is performed, thereby forming an alignment film of thepresent invention. Further, as a reference, a substrate without thealignment film is prepared.

First, as for a sample without the alignment film, the total reflectionangle θ_(S2) at the boundary surface between air and the underlyinglayer is measured when light is incident from the glass substrate, andthe refractive index of the uppermost portion of the underlying layer isobtained by n_(S2)=1/sin θ_(S2) in terms of Snell's law. Next, a samplewith the alignment film is prepared, and a carbon sputter coating isperformed on the surface of the alignment film, thereby preventing thereflection of the light at the uppermost portion of the alignment film.Here, light is incident from the substrate side, and the reflected lightis measured at the boundary surface between the underlying layer and thealignment film. At this time, a P-polarization component parallel to thelight incident surface and an S-polarization component perpendicular tothe light incident surface are separately measured, and Brewster's angleθ_(B1) at which the polarization component p becomes 0 is measured.Here, the refractive index at the lowermost portion of the alignmentfilm may be obtained by n_(F1)=n_(S2) tan θ_(B1). When both are equal toeach other, no reflection occurs.

On the other hand, in order to obtain the refractive index n_(F1) at theuppermost portion of the alignment film, a sample with the alignmentfilm is prepared, the reflection at the boundary surface is prevented byperforming a carbon sputter coating on a bottom surface of a glasssubstrate on the side of ITO, and the reference glass substrate havingthe refractive index n_(RG) is bonded to the surface of the alignmentfilm by matching oil. Light is incident from the reference glasssubstrate, and Brewster's angle θ_(RG) at which the polarizationcomponent p becomes 0 is measured by the same method. At this time, therefractive index at the uppermost portion of the alignment film may beobtained by n_(F2)=n_(RG) tan θ_(RG). When both are equal to each other,no reflection occurs.

Hereinafter, the present invention will be described in more detail byusing the examples, but the technical scope of the present invention isnot limited to the examples below.

First Example

First, polyimide having a chemical structure shown in the followingchemical formula (1) was variously synthesized as an alignment film.

X in the chemical structure shown in the above-described chemicalformula (1) includes two types of the following chemical formulae (X1)and (X2).

In the synthesizing of polyimide, polyimide having the above-describedchemical structure may be formed when pyromellitic acid is used as a rawmaterial.

In the synthesizing of polyimide, polyimide having the above-describedchemical structure may be formed when cyclobutane-tetracarboxylic acidis used as a raw material.

Further, X in the chemical structure shown in the above-describedchemical formula (1) may desirably adopt the following chemical formula(X3).

In the synthesizing of polyimide, polyimide having the above-describedchemical structure may be formed when 1,3dimethyl-cyclobutane-tetracarboxylic acid is used as a raw material.

Further, A in the chemical structure shown in the above-describedchemical formula (1) includes nine types of the following chemicalformulae (Aa1) to (Ac2).

The chemical frame A expressed by the chemical formulae (Aa1) to (Aa5)has a molecular frame having high liquid crystal alignment capability.Further, the chemical frame A expressed by the chemical formulae (Ab1)to (Ab2) includes a sulfur atom used to have a high refractive index.Further, as a molecular frame used to have polarity, the examplesexpressed by the chemical formulae (Ac1) to (Ac2) were selected.

Polyimide having the combination above was synthesized with polyamideacid as a precursor before imidization in accordance with the existingsynthesis method. As for polyimide having a normal refractive index andno polarity, tetracarboxylic acid as a raw material and phenylenediaminewere synthesized at the ratio of X:A=1:1 by the combination of X═X1, X2and A=Aa1 to Ac5, whereby polyamic acid was synthesized (polyimideformed by the composition will be referred to as a symbol P-X-A, butX=1, 2, and A=a1 to a5). Further, as for polyimide having a highrefractive index and polarity, tetracarboxylic acid as a raw materialand phenylenediamine were synthesized at the ratio of X:(A+A′)=1:1 bythe combination of X═X1, X2, A=Ab1, Ab2, and A′=Ac1, Ac2, wherebypolyamic acid was synthesized. When the ratio between A and A′ was setto A+A′=100%, samples were manufactured in an order of A=0, 20, 40, 60,80, 100% (polyimide formed by the composition will be referred to as asymbol P-X-AA′-n, but X=1, 2, A=b1 to b2, A′=c1 to c2, and n=0, 20, 40,60, 80, 100). The molecular weight of the obtained polymer wasdetermined by a GPC method, and a number average molecular weightexpressed in terms of styrene was obtained. The obtained polyamide acidused the resultant dissolved in a mixed solvent of N-methylpyrrolidone(NMP), γ-butyl lactone (GBL), and butyl cellosolve (BC) as an alignmentfilm varnish.

Next, a sample for evaluating the material property of the alignmentfilm was manufactured by the following procedures. An ITO transparentelectrode glass substrate was used as a substrate, and was sufficientlycleaned in advance and irradiated by UV/O₃. The alignment film varnishwas spin-coated thereon by spin-coating, was immediately and temporarilydried at 80° C. for one minute, and imidized by baking at 230° C. forone hour. Here, the rotation speed of the spin-coating and theconcentration of the varnish were selected so that the film thicknessbecame about 200 nm after the imidization by baking. Further, therefractive index of the obtained polyimide thin film was evaluated byusing an Abbe refractometer, and the average value measured in the rangeof a wavelength from 380 to 780 nm using spectroanalysis of a whitelight source was set as the refractive index of the thin film.

Tables 1A to 1E show the molecular weight and the refractive index ofthe obtained polyimide thin film. In polyimide that has a normalrefractive index, but does not have a polarity group, the refractiveindex is from about 1.5 to about 1.6. However, in polyimide that has ahigh refractive index and a polarity group, the refractive index is 1.7or more.

TABLE 1A POLYMER MOLECULAR WEIGHT REFRACTIVE INDEX P-1-a1 15,600 1.610P-1-a2 16,100 1.605 P-1-a3 16,200 1.600 P-1-a4 15,200 1.594 P-1-a515,900 1.588 P-2-a1 15,100 1.596 P-2-a2 15,300 1.592 P-2-a3 16,400 1.587P-2-a4 15,700 1.579 P-2-a5 16,400 1.569

TABLE 1B POLYMER MOLECULAR WEIGHT REFRACTIVE INDEX P-1-b1c1-0 15,5001.604 P-1-b1c1-20 15,000 1.634 P-1-b1c1-40 16,200 1.665 P-1-b1c1-6015,700 1.695 P-1-b1c1-80 16,200 1.725 P-1-b1c1-100 15,200 1.753P-2-b1c1-0 15,800 1.583 P-2-b1c1-20 15,300 1.616 P-2-b1c1-40 15,0001.649 P-2-b1c1-60 15,700 1.684 P-2-b1c1-80 16,300 1.716 P-2-b1c1-10015,600 1.748

TABLE 1C POLYMER MOLECULAR WEIGHT REFRACTIVE INDEX P-1-b1c2-0 15,1001.734 P-1-b1c2-20 15,700 1.738 P-1-b1c2-40 15,400 1.742 P-1-b1c2-6015,900 1.745 P-1-b1c2-80 15,000 1.749 P-1-b1c2-100 15,200 1.753P-2-b1c2-0 15,900 1.583 P-2-b1c2-20 15,000 1.616 P-2-b1c2-40 16,2001.650 P-2-b1c2-60 15,800 1.683 P-2-b1c2-80 15,400 1.717 P-2-b1c2-10015,600 1.748

TABLE 1D POLYMER MOLECULAR WEIGHT REFRACTIVE INDEX P-1-b2c1-0 15,5001.604 P-1-b2c1-20 16,200 1.634 P-1-b2c1-40 16,200 1.664 P-1-b2c1-6015,800 1.694 P-1-b2c1-80 16,200 1.723 P-1-b2c1-100 16,300 1.753P-2-b2c1-0 15,800 1.583 P-2-b2c1-20 15,700 1.616 P-2-b2c1-40 15,0001.649 P-2-b2c1-60 16,000 1.682 P-2-b2c1-80 15,000 1.715 P-2-b2c1-10015,700 1.746

TABLE 1E POLYMER MOLECULAR WEIGHT REFRACTIVE INDEX P-1-b2c2-0 15,1001.734 P-1-b2c2-20 15,100 1.738 P-1-b2c2-40 16,100 1.742 P-1-b2c2-6015,100 1.745 P-1-b2c2-80 16,200 1.749 P-1-b2c2-100 16,300 1.753P-2-b2c2-0 15,900 1.583 P-2-b2c2-20 15,500 1.616 P-2-b2c2-40 15,3001.649 P-2-b2c2-60 16,000 1.681 P-2-b2c2-80 15,100 1.714 P-2-b2c2-10015,700 1.746

Next, such polyimides were combined to form blend polymer, and theresultant was subjected to spin coating, drying, and imidization bybaking in the same procedure. Next, the result will be described. Thepolyimide having a low refractive index component and the polyimidehaving a high refractive index component were mixed with each other atthe weight ratio of 1:1 in a varnish state, and the mixture was used asa varnish for blend polymer. In the same condition, the thickness of thefilm was thickened, the surface state thereof was first observed by SEM,AFM, or the like, and a phase separation was performed. Even in thisstate, the result in which a sea-island structure was observed wasdetermined as a inhomogeneous state (symbol: I), and the result in whicha domain was not particularly observed was determined as a homogeneousstate (symbol: H). Next, as for the thin film in a homogeneous state, anauger spectral analysis was performed in the depth direction, and as inthe profile of the concentration of sulfur atom S shown in FIG. 5B, thecase where the concentration smoothly decreases from the ITO to thesurface of the film (the case where the concentration continuously andmonotonically decreases) was determined as a gradient state (symbol: G).Tables 3 to 5 show the evaluation result.

In the evaluation result, the material of each table has the followingcombinations (1) to (5).

(1) Table 1: A to D: low refractive index component=P-1-a1 to a5 andP-2-a1 to a5 as common material

(2) Table 2: high refractive index component=P-1-b1c1-0 to 100,P-2-b1c1-0 to 100

(3) Table 3: high refractive index component=P-1-b1c2-0 to 100,P-2-b1c2-0 to 100

(4) Table 4: high refractive index component=P-1-b2c1-0 to 100,P-2-b2c1-0 to 100

(5) Table 5: high refractive index component=P-1-b2c2-0 to 100,P-2-b2c2-0 to 100

TABLE 2A POLYMER P-1-a1 P-1-a2 P-1-a3 P-1-a4 P-1-a5 P-1-b1c1-0 I I H H HP-1-b1c1-20 I I H H H P-1-b1c1-40 I I H H H P-1-b1c1-60 I H H H HP-1-b1c1-80 H H H H H P-1-b1c1-100 H H H H H P-2-b1c1-0 I I I H HP-2-b1c1-20 I I H H H P-2-b1c1-40 I I H H H P-2-b1c1-60 I I H H HP-2-b1c1-80 I H H H H P-2-b1c1-100 H H H H H

TABLE 2B POLYMER P-2-a1 P-2-a2 P-2-a3 P-2-a4 P-2-a5 P-1-b1c1-0 I I I H HP-1-b1c1-20 I I H H H P-1-b1c1-40 I I H H H P-1-b1c1-60 I I H H HP-1-b1c1-80 H H H HG HG P-1-b1c1-100 H H H H H P-2-b1c1-0 I I H H HP-2-b1c1-20 I I H H H P-2-b1c1-40 I I H H H P-2-b1c1-60 I H H H HP-2-b1c1-80 H H H H HG P-2-b1c1-100 H H H H H

TABLE 3A POLYMER P-1-a1 P-1-a2 P-1-a3 P-1-a4 P-1-a5 P-1-b1c2-0 I I I IHG P-1-b1c2-20 I I I HG HG P-1-b1c2-40 I I HG HG HG P-1-b1c2-60 I I HGHG HG P-1-b1c2-80 I HG HG HG HG P-1-b1c2-100 H H H H H P-2-b1c2-0 I I II I P-2-b1c2-20 I I I H H P-2-b1c2-40 I I H H H P-2-b1c2-60 I I H H HP-2-b1c2-80 I I H H H P-2-b1c2-100 H H H H H

TABLE 3B POLYMER P-2-a1 P-2-a2 P-2-a3 P-2-a4 P-2-a5 P-1-b1c2-0 I I I I IP-1-b1c2-20 I I I I HG P-1-b1c2-40 I I I HG HG P-1-b1c2-60 I I HG HG HGP-1-b1c2-80 I HG HG HG HG P-1-b1c2-100 H H H H H P-2-b1c2-0 I I I I HP-2-b1c2-20 I I I H H P-2-b1c2-40 I I H H H P-2-b1c2-60 I I H H HP-2-b1c2-80 I H H H HG P-2-b1c2-100 H H H H H

TABLE 4A POLYMER P-1-a1 P-1-a2 P-1-a3 P-1-a4 P-1-a5 P-1-b2c1-0 I I H H HP-1-b2c1-20 I I H H H P-1-b2c1-40 I I H H H P-1-b2c1-60 I H H H HP-1-b2c1-80 H H H H H P-1-b2c1-100 H H H H H P-2-b2c1-0 I I I H HP-2-b2c1-20 I I I H H P-2-b2c1-40 I I H H H P-2-b2c1-60 I I H H HP-2-b2c1-80 I H H H H P-2-b2c1-100 H H H H H

TABLE 4B POLYMER P-2-a1 P-2-a2 P-2-a3 P-2-a4 P-2-a5 P-1-b2c1-0 I I I H HP-1-b2c1-20 I I I H H P-1-b2c1-40 I I H H H P-1-b2c1-60 I I H H HP-1-b2c1-80 I H H HG HG P-1-b2c1-100 H H H H H P-2-b2c1-0 I I H H HP-2-b2c1-20 I I I H H P-2-b2c1-40 I I H H H P-2-b2c1-60 I I H H HP-2-b2c1-80 I H H H HG P-2-b2c1-100 H H H H H

TABLE 5A POLYMER P-1-a1 P-1-a2 P-1-a3 P-1-a4 P-1-a5 P-1-b2c2-0 I I I IHG P-1-b2c2-20 I I I I HG P-1-b2c2-40 I I HG HG HG P-1-b2c2-60 I HG HGHG HG P-1-b2c2-80 H HG HG HG HG P-1-b2c2-100 H H H H H P-2-b2c2-0 I I II I P-2-b2c2-20 I I I I H P-2-b2c2-40 I I I H H P-2-b2c2-60 I I I H HP-2-b2c2-80 I I H H H P-2-b2c2-100 H H H H H

TABLE 5B POLYMER P-2-a1 P-2-a2 P-2-a3 P-2-a4 P-2-a5 P-1-b2c2-0 I I I I IP-1-b2c2-20 I I HG HG HG P-1-b2c2-40 I I HG HG HG P-1-b2c2-60 I I HG HGHG P-1-b2c2-80 HG HG HG HG HG P-1-b2c2-100 H H H H H P-2-b2c2-0 I I I IH P-2-b2c2-20 I I I I H P-2-b2c2-40 I I I H H P-2-b2c2-60 I I H H HP-2-b2c2-80 I H H H HG P-2-b2c2-100 H H H H H

When various combinations of the materials are searched in this manner,the condition (symbol: HG) capable of realizing the grated concentrationdistribution may be found. However, any condition may not be found ineach Table (for example, Table 2A) or the condition may be found veryfrequently (for example, Table 3A). That is, the alignment film having adesired grated concentration distribution is difficult to be realized bya simple attempt in which the high refractive index component and thelow refractive index component are combined, or a non-polarity componentand a high-polarity component are combined.

Next, the IPS type liquid crystal display element is manufactured byusing the polyimide alignment film at the condition capable of realizingthe grated concentration distribution, and the evaluation result of thetransmissivity thereof will be described. As for the polymer of X-X1, arubbing alignment treatment was performed by using a rubbing cloth madeof rayon in the condition that the rotation speed was 1500 rpm, thetransfer speed was 32.5 mm/min, the cutting depth was 0.4 mm, and thetemperature was room temperature under the presence of the atmosphere.Further, as for the polymer of X═X2, photo-alignment treatment wasperformed in the condition that polarized UV light was emitted to thesurface of the substrate in the perpendicular direction, light having awavelength from 230 to 300 nm was selectively emitted from alow-pressure mercury light source, the temperature of the substrate was200° C. during the emission, and the emission energy was 2 J.

The liquid crystal panel is manufactured by the same process in thegeneral manufacturing process except that the alignment film is formedof the alignment film material of the present invention. For example, inthe case of the representative manufacturing method of the IPS typeliquid crystal display device, the active matrix substrate 6 and thecounter substrate 7 subjected to the alignment treatment in advance arebonded to each other, and a liquid crystal material is enclosedtherebetween to assemble a cell. However, at this time, the initialalignment direction of the alignment film 606 of the active matrixsubstrate 6 and the initial alignment direction of the alignment film705 of the counter substrate 7 are set to be substantially parallel toeach other.

Further, as the liquid material to be enclosed, for example, a nematicliquid crystal composition A is used in which dielectric anisotropy Δ∈is positive, the value thereof is 10.2 (1 kHz, 20° C.), the refractiveindex anisotropy Δn is 0.075 (wavelength of 590 nm, 20° C.), the torsionelastic constant K2 is 7.0 pN, the nematic-isotropic phase transitiontemperature T (N−I) is about 76° C., and the specific resistance is1×10⁺¹³ Ωcm. In this case, the active matrix substrate 6 and the countersubstrate 7 may be bonded to each other so that the thickness (cell gap)of the liquid crystal layer 11 a is substantially equal to the height ofthe columnar spacer 10 such as to be, for example, 4.2 μm. Theretardation (Δn·d) of the liquid crystal display panel 1 manufactured inthis condition is about 0.31 μm. It is desirable that the retardationΔn·d is 0.2 μm≦Δn·d≦0.5 μm, and when the retardation exceeds this range,a problem arises in that a white display occurs.

When the active matrix substrate 6 and the counter substrate 7 arebonded to each other and a liquid crystal material is enclosedtherebetween, for example, the unnecessary portions (margin portions) ofthe outer peripheries of the glass substrates 601 and 701 are cut to beeliminated, and polarizers 9 a and 9 b are bonded thereto. When thepolarizers 9 a and 9 b are bonded to each other, the polarizationtransmission axis of one polarizer is set to be substantially parallelto the initial alignment directions of the alignment film 606 of theactive matrix substrate 6 and the alignment film 705 of the countersubstrate 7, and the polarization transmission axis of the otherpolarizer is set to be perpendicular thereto. Subsequently, when thefirst driving circuit 2, the second driving circuit 3, the controlcircuit 4, the backlight 5, and the like are connected to each other tobe formed as a module, the liquid crystal display device having theliquid crystal display panel 1 of the first example is obtained.

Incidentally, in the liquid crystal display panel 1 of the firstexample, when a difference in potential between the pixel electrode PXand the common electrode CT is small, a dark display (low-luminancedisplay) is performed. When a difference in potential between the pixelelectrode PX and the common electrode CT is large, a bright display(high-luminance display) is performed. In this manner, the liquidcrystal panel has a normally-closed characteristic. Even in the case ofany type of the liquid crystal display device, each type is manufacturedby the general process so that a dark display and a bright display maybe performed.

As for the transmissivity of the liquid crystal display element, a whitecold cathode tube backlight was used as a light source, the lighttransmitted through the liquid crystal display element was input to theactinometer without dividing the light, and the transmissivity of thelight was expressed as a percentage. Further, for the comparison, thetransmissivity was measured when each of the single low refractive indexcomponent and the single high refractive index component formed thealignment film. Table shows the evaluation result. In any combination,the transmissivity of the grated concentration film is improved comparedwith the case of a single low refractive index and a single highrefractive index.

TABLE 6 LOW REFRACTIVE HIGH REFRACTIVE TRANSMISSIVITY (%) INDEXCOMPONENT INDEX COMPONENT SINGLE LOW SINGLE HIGH GRATED REFRACTIVEREFRACTIVE REFRACTIVE REFRACTIVE CONCENTRATION SYMBOL INDEX SYMBOL INDEXINDEX INDEX FILM P-2-a4 1.579 P-1-b1c1- 1.725 83.9 81.4 94.2 80 P-1-a31.600 P-1-b1c2- 1.745 89.0 89.1 95.5 60 P-2-a5 1.569 P-2-b2c1- 1.71578.6 80.8 86.8 80 P-1-a2 1.605 P-1-b2c2- 1.745 86.8 87.5 98.0 60

As described above, in the polyimide alignment film having a gratedrefractive index, there is an effect of improving the transmissivity ofthe liquid crystal display device.

Second Example

Next, the FFS type liquid crystal display element shown in FIG. 3 ismanufactured by using the alignment film material shown in the firstexample, and the evaluation result of the transmissivity will bedescribed. The element structure of the FFS type is similar to that ofthe IPS type, only one of the upper and lower underlying substrates isprovided with the pixel electrode PX and the common electrode CT, andthe liquid crystal rotates within the plane in accordance with thepresence of the electric field. Accordingly, the initial alignment statewithout the application of the electric field is also equal to that ofthe IPS type, the direction may be equal to the alignment direction tobe formed in the alignment film 606 (and 705), and the liquid crystal tobe used may be the liquid crystal having positive dielectric anisotropyΔ∈.

Table 7 shows the evaluation result. In any combination, thetransmissivity of the grated concentration film is improved comparedwith the case of a single low refractive index and a single highrefractive index.

TABLE 7 LOW REFRACTIVE HIGH REFRACTIVE TRANSMISSIVITY (%) INDEXCOMPONENT INDEX COMPONENT SINGLE LOW SINGLE HIGH GRATED REFRACTIVEREFRACTIVE REFRACTIVE REFRACTIVE CONCENTRATION SYMBOL INDEX SYMBOL INDEXINDEX INDEX FILM P-2-a4 1.579 P-1-b1c1- 1.725 83.4 84.5 89.2 80 P-1-a31.600 P-1-b1c2- 1.745 85.6 85.9 95.4 60 P-2-a5 1.569 P-2-b2c1- 1.71578.6 79.2 84.9 80 P-1-a2 1.605 P-1-b2c2- 1.745 87.1 89.6 96.8 60

As described above, in the polyimide alignment film having a gratedrefractive index, there was an effect of improving the lighttransmissivity of the liquid crystal display device.

Third Example

Next, the VA type liquid crystal display element shown in FIG. 4 will bemanufactured by using the alignment film material shown in the firstexample, and the evaluation result of the light transmissivity will bedescribed. The VA type is different from the IPS type or the FFS type inthat the upper and lower underlying substrates are provided with thepixel electrode PX and the common electrode CT, a liquid crystalmaterial for a VA type having negative dielectric anisotropy Δ∈ is used,and the alignment treatment needs to be performed so that the liquidcrystal molecule is substantially perpendicular to the surface of thesubstrate at the initial alignment state where there is no applicationof an electric field. For this reason, it is difficult to use a generalrubbing treatment. Here, as shown in Technical document 4 below,photo-alignment treatment was performed by emitting polarized UV lightin a gradient direction.

Technical document 4: P. Gass, H. Stevenson, R. Bay, H. Walton, N.Smith, S. Terashita, and M. illin: Pattern Optical Alignment ForVertical Alignment LCD: Sharp Technical Journal Vol. 85 (2003) 24-29

Table 8 shows the evaluation result. In any combination, thetransmissivity of the grated concentration film is improved comparedwith the case of a single low refractive index and a single highrefractive index.

TABLE 8 LOW REFRACTIVE HIGH REFRACTIVE TRANSMISSIVITY (%) INDEXCOMPONENT INDEX COMPONENT SINGLE LOW SINGLE HIGH GRATED REFRACTIVEREFRACTIVE REFRACTIVE REFRACTIVE CONCENTRATION SYMBOL INDEX SYMBOL INDEXINDEX INDEX FILM P-2-a4 1.579 P-1-b1c1- 1.725 83.0 84.0 89.1 80 P-1-a31.600 P-1-b1c2- 1.745 87.8 86.2 96.3 60 P-2-a5 1.569 P-2-b2c1- 1.71577.1 80.2 85.6 80 P-1-a2 1.605 P-1-b2c2- 1.745 89.0 89.9 96.9 60

As described above, in the polyimide alignment film having a gratedrefractive index, there is an effect of improving the lighttransmissivity of the liquid crystal display device.

Further, in the first to third examples, plural types of polyimides wereused as the organic compounds forming the alignment film, but forexample, the alignment film may be formed by mixing other polymers oronly by an organic compound except for polyimide.

While there have been described what are at present considered to becertain embodiments of the present invention, it will be understood thatvarious modifications may be made thereto, and it is intended that theappended claim cover all such modifications as fall within the truespirit and scope of the present invention.

1. A liquid crystal display device comprising: first and secondsubstrates, at least one of which is transparent; a liquid crystal layerwhich is disposed between the first and second substrates; an electrodegroup which is formed on at least one of the first and second substratesand applies an electric field to the liquid crystal layer; a pluralityof active elements which is connected to the electrode group; analignment film which is disposed on at least one of the first and secondsubstrates and has one surface contacting the liquid crystal layer; andan underlying layer which is disposed on at least one of the first andsecond substrates and contacts the other surface of the alignment film,wherein the alignment film includes an organic compound, wherein therefractive index of the alignment film monotonically increases from aboundary surface between the alignment film and the liquid crystal layerto a boundary surface between the alignment film and the underlyinglayer, and wherein the minimum refractive index n_(LC) of the liquidcrystal layer, the refractive index n_(F2) of the alignment film at theboundary surface between the liquid crystal layer and the alignmentfilm, the refractive index n_(F1) of the alignment film at the boundarysurface between the alignment film and the underlying layer, and therefractive index n_(S2) of the underlying layer have a relationship ofan equation (I) in which n_(LC)≦n_(F2)<n_(F1)≦n_(S2).
 2. The liquidcrystal display device according to claim 1, wherein the underlyinglayer is a transparent layer through which visible light is transmitted,wherein the electrode group includes a common electrode and a pixelelectrode, and wherein a surface opposite to a surface contacting thealignment film in the underlying layer contacts any one of the commonelectrode and the pixel electrode.
 3. The liquid crystal display deviceaccording to claim 2, wherein the refractive index n_(S2) of theunderlying layer and the refractive index n_(PE) of the one electrode atthe boundary surface between the underlying layer and the one electrodehave a relationship of an equation (II) in which |n_(S2)−n_(PE)|≦0.1. 4.The liquid crystal display device according to claim 2, wherein a partof the surface opposite to the surface contacting the alignment film inthe underlying layer contacts the one electrode and the other partthereof contacts an interlayer isolation film, and wherein therefractive index n_(S2) of the underlying layer and the refractive indexn_(LI) of the interlayer isolation film have a relationship of anequation (III) in which |n_(S2)−n_(LI)|≦0.1.
 5. The liquid crystaldisplay device according to claim 1, wherein the electrode groupincludes a common electrode and a pixel electrode, wherein theunderlying layer includes an interlayer isolation film contacting asurface opposite to a surface contacting the liquid crystal layer in thealignment film and any one of the common electrode and the pixelelectrode provided in a part of a surface on the side of the alignmentfilm in the interlayer isolation film and protruding toward thealignment film, wherein the alignment film is formed on the underlyinglayer, and wherein the film thickness of the alignment film is largerthan that of the one electrode.
 6. The liquid crystal display deviceaccording to claim 2, wherein the refractive index of the surface of theone electrode is lower than the internal refractive index.
 7. The liquidcrystal display device according to claim 1, wherein the alignment filmhas an in-plane optical anisotropy.
 8. The liquid crystal display deviceaccording to claim 7, wherein the direction of the in-plane opticalanisotropic axis of the alignment film is equal to the alignmentregulation direction of the liquid crystal layer.
 9. The liquid crystaldisplay device according to claim 1, wherein the alignment film includesa plurality of types of the organic compounds.
 10. The liquid crystaldisplay device according to claim 1, wherein the alignment film includesa plurality of types of the organic compounds having differentpolarities.
 11. The liquid crystal display device according to claim 1,wherein the alignment film includes a plurality of types of the organiccompounds having different refractive indexes.
 12. The liquid crystaldisplay device according to claim 11, wherein in the plurality of typesof organic compounds forming the alignment film, the refractive index inthe visible range of the organic compound having the highest refractiveindex is 1.7 or more.
 13. The liquid crystal display device according toclaim 1, wherein the alignment film is formed of polyimide usingpolyamic acid ester as a precursor.
 14. The liquid crystal displaydevice according to claim 1, wherein the alignment film obtains liquidcrystal alignment capability by photo-alignment treatment.
 15. Theliquid crystal display device according to claim 1, wherein thealignment film obtains liquid crystal alignment capability by a rubbingtreatment.
 16. The liquid crystal display device according to claim 1,wherein an area obtaining liquid crystal alignment capability of thealignment film is within a range of 20 nm from a surface of thealignment film.
 17. The liquid crystal display device according to claim1, wherein compounds forming the alignment film are cross-linked to eachother after the alignment film obtains liquid crystal alignmentcapability.
 18. The liquid crystal display device according to claim 1,wherein the coating ratio with respect to a display area of thealignment film is 50% or more.